Fine‐scale reconstruction of pelagic fish migration by iso‐logging of eye lens

Understanding lifetime space use by pelagic animals is pivotal for ecology and fisheries management, but electronic tags are costly, labour‐intensive and rarely able to capture juvenile movement. We implemented an iso‐logging workflow that converts stable isotope chronologies in eye lenses into continuous migration tracks, and demonstrate its application to skipjack tuna ( Katsuwonus pelamis ). The δ 13 C and δ 15 N of muscle from 1122 skipjack were combined with oceanographic predictors in a geostatistical model to produce isoscapes of the western Pacific. Then, a newly developed dry‐and‐scrape protocol was applied to eye lenses from 33 adult skipjack and yielded high‐resolution δ 13 C and δ 15 N time series. These isotope ratios were normalised to muscle–lens offsets and ontogenetic enrichment, and a Bayesian state‐space model was used to estimate lifetime migration tracks for each individual. Ensemble reconstructions exposed two migration habits: most fish caught in the western tropical Pacific were lifelong residents, whereas several North Pacific fish had migrated there from tropical nurseries. This provides the first direct evidence of partial migration in a highly migratory pelagic fish. We show that the reconstruction of migration by iso‐logging is a powerful approach for reconstructing the lifetime migration history of pelagic fish. This approach could be applied to a variety of other marine organisms, possibly providing a breakthrough for behavioural ecology studies of marine ecosystems.